It is known to use series capacitor equipment in electric power transmission lines to increase the transmission capacity of the lines. Equipment of this kind is particularly used in high voltage transmission lines for power transmission over long distances.
To protect a series capacitor against overvoltages, it is known to use varistors, spark gaps and electric switches in various combinations. Such overvoltage protective means are previously known from, for example, Swedish published patent application No. 358,509 and U.S. patent specification Nos. 4,028,592, 4,625,254 and 4,652,963. Since a series capacitor is often essential for the stability of a power network, it is of importance that it is not shunted other than when it is inevitably necessary to avoid damage caused by overvoltages. Since very high overvoltages may occur across a series capacitor in the case of short circuits or other disturbances in the power network, it is furthermore of the utmost importance to have an overvoltage protective means which, with great reliability, starts to function when this is necessary to protect the capacitor against damage.
In conventional series capacitor equipment, the above requirements are fulfilled to a satisfactory extent by prior art overvoltage protective means of, for example, the types described in the above-mentioned publications.
However, it has been proposed to design series capacitor equipment with a variable capacitance. By means of such equipment in a power network, the distribution of the load flux between the different branches of the network could be controlled in a rapid and simple manner. Such capacitor equipment would conveniently consist of a fixed part connected in series with a variable part, where the latter part would consist of a plurality of series-connected partial capacitors arranged to be switched in or shunted by means of electromechanical or static switching members.
Providing an overvoltage protective means for a variable series capacitor has proved to involve problems. Arranging one single overvoltage protective means in parallel with the entire series capacitor equipment is not practical since the voltage, at which the protective means is to be activated, varies within wide limits with the number of switched-in capacitor steps. A possible solution would be to arrange a separate overvoltage protective means in parallel with each partial capacitor. Such equipment would, however, become complicated and expensive, and it has proved to be difficult to provide suitable spark gaps for the relatively low voltages that will typically occur across each partial capacitor.